Don't confuse state-of-charge (SOC) with capacity. It's perfectly possible to have a fully charged battery (100% SOC) showing 12.6 volts after a resting period, but with greatly reduced capacity due to sulfation, corrosion, contamination, plate damage, stratification, and similar ills which -- over time -- will kill a battery for all practical purposes.

BOTTOM LINE: resting voltage is not an indication of battery capacity.

There are several ways to test battery capacity...several posts mentioned a "battery tester" like those in garages or WalMart. These are fairly good, though not definitive. Electronic testers like the Micronics series are also pretty good, but very pricey ($600 class).

The best way to test capacity is to fully charge the battery, then apply a constant load calculated at the battery's 20-hour rating. E.g., for a bank of two T-105's in series (total 225AH @ the 20-hour rate), you'd put a resistive load of about 11 amps and see how long it takes before the battery bank reaches 10.5 volts (the effective point of depletion).

My suggestion: toss the batteries you have and start next season with new ones. Treat them right. Read and learn about battery care over the winter, get a good multi-stage charger, and keep them charged. For a small boat, even a portable charger like the West Marine ones now on sale would do the trick. The 30A model is particularly a good buy @ $89, as it has an equalization cycle, and would be the right size for two T-105s.

I am suprised that no one has mentioned checking the specific gravity of the individual cells. After fully charging the battery (it is fully charged when after 3 hrs the specific gravity remains unchanged), remove the charger and let it sit for a couple of hrs(internally stablize). Individual cells should test at 1.275, this is a very general number for northern climates. As an example, if all the cells test at 1.275 , except for one cell that tests 1.210. The lower reading indicates a weak cell. You may need to equalize it if it is a deep cycle . If a regular wet cell this is a warning sign that it is dying. I check my batts every 6 months and record amt of distilled water added and SG of each cell. A quick way to check your batts load capacity, is to disable your eng so it won't start. Use your meter to read the volts on your batt as you crank it for about 10 sec. It should drop to around 10v as soon as it starts to crank. If it is a good batt it will maintain this voltage. If it drops below 9.5 it is junk. Good luck

"How can I tell if the batteries have been compromised "
That's easy, they have been. You'll find that pretty much ALL the battery makers in the US will gladly answer the phone and provide free tech support, even if you are just shopping around. And their answers all tend to be the same, even if the details vary a little.
All of them will tell you that a wet cell takes permanent damage, permanent capacity loss, from sulphation after 90 days. The compounds that form precipitate out of solution and there is no way to get them back into solution, so the battery loses SOME capacity, some will say after 30 or 60 days, all will say by 90.
They also all will say that every time you deep cycle a battery, no matter what kind of battery it is, it takes some damage. Some claim their SLI batteries will be permanently ruined and worthless in as few as 6 deep cycles. Others say their deep cycle batteries can take 50-300 or more deep cycles. But again--they all agree that killing a battery, totally discharging it and then letting it sit discharged, will permanently reduce the capacity.

So the only real question is whether your batteries are so badly damaged as to be junk, or have some useful life left in them.

And the only real way to find that out, is to put a charge on them, cycle them a couple of times, and load test them to see what capacity is left in them. I wouldn't bother putzing around with a store, where they may fast-charge them and damage them further, or prefer to scrap them to sell you new ones. I'd buy a charger of some kind, even a 6-amp car charger for $25, put a slow charge on them one at a time, let them sit overnight, check 'em out, repeat twice. You'll know whether they are or aren't any good, and you'll only have spent $25 for a charger you can always use on the car anyhow. Just make sure the charger CAN be used with a dead battery, because many of the new electronic ones are designed so that they will not even begin to charge a battery under about 10V, as a safety measure. (Supposedly.)

Got a neighbor that can lend you a charger?

The distilled water, btw, is about $1.50/gallon at the supermarket. No big deal, and if you can't find distilled used "deionized" in the laundry section.

Just make sure the charger CAN be used with a dead battery, because many of the new electronic ones are designed so that they will not even begin to charge a battery under about 10V, as a safety measure.

This is the reason why you save those old but perfectly usable non-circuit-board-containing automobile battery chargers. All that they normally need is the cable clamps pulled and the connections cleaned up and then reattached.

What do the battery experts here (I'm not one of them!) think about the BatteryMinder? It's a trickle charger that claims to desulphate through pulses of voltage. I have one, and I've used it on batteries than ran very low. The batteries seem to come back to life and have seen serious use afterwards -- but as mentioned above, their true capacity may be diminished.

It's really simple, to buy new batteries or not?
Do you enjoy the prospect of dealing with a dead battery during a cruise that you have waited all year for?
If thats your kind of fun then keep on using them till death.

Don't confuse state-of-charge (SOC) with capacity. It's perfectly possible to have a fully charged battery (100% SOC) showing 12.6 volts after a resting period, but with greatly reduced capacity due to sulfation, corrosion, contamination, plate damage, stratification, and similar ills which -- over time -- will kill a battery for all practical purposes.

BOTTOM LINE: resting voltage is not an indication of battery capacity.

There are several ways to test battery capacity...several posts mentioned a "battery tester" like those in garages or WalMart. These are fairly good, though not definitive. Electronic testers like the Micronics series are also pretty good, but very pricey ($600 class).

The best way to test capacity is to fully charge the battery, then apply a constant load calculated at the battery's 20-hour rating. E.g., for a bank of two T-105's in series (total 225AH @ the 20-hour rate), you'd put a resistive load of about 11 amps and see how long it takes before the battery bank reaches 10.5 volts (the effective point of depletion).

My suggestion: toss the batteries you have and start next season with new ones. Treat them right. Read and learn about battery care over the winter, get a good multi-stage charger, and keep them charged. For a small boat, even a portable charger like the West Marine ones now on sale would do the trick. The 30A model is particularly a good buy @ $89, as it has an equalization cycle, and would be the right size for two T-105s.

Bill

Thanks for advice ...bill and others.

So..more stupid questions .....this charger you speak of...how would it be set up with my shore power? Permanently installed in some (mystery to me) location between the shore powerplug and the batteries?

ps..apparantly one of the few complaints people have of the old Tartan 34's is limited battery space....dergon_gf (the engineer) is concerned that we won't be able to fit 2 group 31s